The development of resistance to chemotherapeutic agents by malignant cells remains a significant problem in successful cancer therapy. In vitro analysis of a mechanism which simultaneously conveys resistance to multiple chemotherapeutic agents has been associated with amplification of a specific region of the genome. The techniques of molecular biology, cell biology and recombinant retroviral vectors will be utilized in characterizing this amplified region and the genes overexpressed in multidrug resistant cell lines. Molecular probes from this amplified region will be used to advance our understanding the chemotherapeutic protocols of the future. The practical application of multidrug resistance genes transduced into hematopoetic stem cells will expand our knowledge of hematopoesis and represents the first step towards utilizing hematopoetic stem cells resistant to chemotherapeutic agents as an integral component of chemotherapeutic regimens and bone marrow transplantation. The specific aims of these studies will be: 1) characterization of the cDNA clones to the gene(s) responsible for multidrug resistance, 2) characterization of the protein(s) encoded by the cDNA clones which are responsible for multidrug resistance, 3) characterization of the mechanism of amplification of multidrug resistance genes, 4) evaluation of children being treated for acute lymphoblastic leukemia for the development of amplification of multidrug resistance gene(s) and its relationship to chemotherapeutic failures, 5) development of in vivo models for the direct analysis of the development of multidrug resistance at the molecular level, 6) construction of a retroviral vector containing the gene responsible for multidrug resistance, and 7) selection in vivo of hematopoetic stem cells carrying the multidrug resistant gene to develop model systems for the analysis of hematopoesis and protection of bone marrow from chemotherapeutic agents.